docs: def PartialDynamicSystem(self, ieq, variable):
          """
          returns dynamical system blocks associated to output variable
          """
          if ieq == 0:
              # w2=Rw1
              if variable == self.physical_nodes[0].variable:
                  # w1=w2/R
                  return[Gain(self.physical_nodes[1].variable, variable, 1/self.ratio)]
              elif variable == self.physical_nodes[1].variable:
                  # w2=Rw1
                  return[Gain(self.physical_nodes[0].variable, variable, self.ratio)]
          elif ieq == 1:
              # C1=-RC2
              if variable == self.variables[0]:
                  # C1=-RC2
                  return[Gain(self.variables[1], variable, -self.ratio)]
              elif variable == self.variables[1]:
                  # C2=-C1/R
                  return[Gain(self.variables[0], variable, -1/self.ratio)]